CDC notes change in ethnic pattern of H1N1 impact

Oct 22, 2009 (CIDRAP News) – The Centers for Disease Control (CDC) has seen a change in the ethnic distribution of fatal pandemic H1N1 influenza cases, a CDC official reported at a meeting of its vaccine advisory committee today.

In an update on epidemiologic patterns in the pandemic H1N1 outbreak, Lyn Finelli, PhD, head of surveillance for the CDC's Influenza Division, told the Advisory Committee on Immunization Practices (ACIP) that the high-risk groups haven't changed since the spring wave.

However, the impact of the virus on racial groups has changed some since the first wave, she told the group, whose meeting was streamed over the Web. Hispanic people had a higher proportion of deaths in the spring, probably because they had connections to people traveling to and from Mexico, where the outbreak is thought to have started. While the number of deaths in Hispanic people has fallen, since September the rate has risen among African Americans, Finelli said. She said the CDC tracks trends through the Behavioral Risk Factor Surveillance System.

Finelli said CDC data show only two deaths in the Native American population, though some members of the committee said some people who work with tribes have received anecdotal reports of higher death rates in Native Americans, and they voiced concern that state surveillance systems may be undercounting deaths and severe illnesses among the tribes.

Finelli noted that not all tribes report flu data to states and that not all states collect data on tribes the same way. The Council of State and Territorial Epidemiologists recently had a conference call to discuss how to improve flu monitoring in native populations, she said.

Also at the meeting, Nancy Cox, MD, director of the CDC Influenza Division, told the committee that the CDC has analyzed 1,280 pandemic H1N1 isolates, 407 of them from other countries. So far it has conducted complete genetic sequencing of 76 isolates, most of them from severe or fatal cases.

She said though the virus continues to evolve and CDC scientists are detecting minor variations, there is no evidence of reassortment or mutation, which is reassuring, because it points to a close match between the circulating strains and the virus in the vaccine.

The World Health Organization has received reports of 38 oseltamivir-resistant pandemic H1N1 cases, only four of which had no history of exposure to the drug, Cox said. "Exposure to the drug is a primary factor to the resistance marker," she said, adding that researchers still don't know why oseltamivir use can sometimes lead to resistant pandemic H1N1 strains but doesn't seem to be a factor in the development of resistant seasonal H1N1 flu viruses.

A variant of the seasonal H3N2 strain that was circulating in South America this past spring, an A/Perth/16/2009-like virus, has been included in the 2010 Southern Hemisphere flu vaccine, Cox said. Laos and southern China have reported circulation of the H3N2 strain. However, she said the CDC has detected only H3N2 A/Brisbane/10/2007, a component of this season's Northern Hemisphere vaccine, in the few seasonal flu strains that it has isolated so far this fall.

Cox said it's difficult to predict how the US flu season will play out on the basis of the Southern Hemisphere's experience this year. The United States had a spring wave of pandemic H1N1, whereas the Southern Hemisphere's H1N1 epidemic hit during the normal flu season. She pointed out that a map of current virus activity in the United States shows a patchwork of hot spots with plenty of places that haven't been hit hard.

Officials can't predict if the season will peak early, but if it does, "that leaves plenty of time for seasonal strains to circulate if the viruses are successful and the population is susceptible," Cox said.